In a conventional large scale display apparatus which is used for instance as an electric light display board at a baseball field or for displaying advertisement pictures on the roofs or walls of buildings, a number of light bulbs arranged are selectively turned on and off to form pictures. Accordingly, the conventional display apparatus suffers from various problems.
Some of the problems will be described. In the case where light bulbs are used to form pictures, each light bulb emits light when the filament becomes red hot, and therefore the emitted light's color is red or white orange. To this end, it is rather difficult to obtain a large amount of blue or green light from a light bulb. In this light bulb system, in order to modulate the luminance of each picture, the currents applied to the filaments must be controlled on and off or varied. These light bulbs are considerably low in frequency response, lower than 10 Hz. Furthermore, as the applied current is not in linear relation to the intensity of emitted light, the system suffers from a problem that the emitted light's color is changed by the applied current. Thus, it is difficult to apply the system to the display of half-tones and to a color display in which optional light colors are to be composed. In general, light bulbs of 10 W or larger are used in such a large scale display apparatus. Therefore, if several ten of thousands of such light bulbs are arranged, then a considerably large electric power is consumed by the light bulbs and a considerably large amount of heat is generated by them.
In order to overcome these drawbacks, a method has been proposed in which cathode ray tubes are employed as the light sources of a display apparatus. More specifically, in the method, a number of small cathode ray tubes having fluorescent surfaces adapted to single-color lights, red, green and blue lights, are arranged to display a desired picture. If it is assumed that the efficiency of converting electrical energy to optical energy of a light bulb is 10 lm/W, then the conversion efficiency of a cathode ray tube is about 100 lm/W, thus being better by about one order of magnitude than that of the light bulb. In the case where cathode ray tubes are employed, a variety of phosphors adapted to emit various color lights as well as red, green and blue lights are available. Therefore, the light sources can be so designed as to emit desired color light, and are considerably high in frequency response. Accordingly, the use of the cathode ray tube makes it possible to display even a motion picture. Being able to change an emitted light luminance with respect to an input electrical signal with high fidelity, the cathode ray tube is most suitble for reproduction of half-tones. In the case where the cathode ray tubes are employed, the heaters are scarcely consumed when compared with the case where the light bulbs are employed and the currents applied to the filaments of the light bulbs are changed. Therefore, the former case is advantageous in service life.
However, almost all the phosphors of ordinary cathode ray tubes are substantially white, its optical reflection facter is about one, and the face plate of the cathode ray tube is made of transparent glass high in optical transmissivity. As a result, since the sun light may impinge through the face plate upon the phosphor, the color of light emitted from the fluorescent surface of the cathode ray tube becomes rather white. This results in the contrast being sometimes lost. Accordingly, in order overcome to such a difficulty, the display apparatus should be so designed that it provides a sufficient contrast even under sun light to meet the purpose of installation of the display apparatus. In order to meet this requirement, the following method has been proposed. In the method, pigment phosphor obtained by covering phosphor particle with pigment is used so as to allow phosphor to be used which has a color corresponding to the color of light emitted thereby. Alternatively the face plate glass or the vacuum envelope including the face plate glass is colored. The result is that under sun light, the color appears clear and the contrast is high. However, in order to allow these colored glasses to have primary colors, it is necessary to provide three different kinds of glass. This is not economical.